What's improved in Starcraft 2?

What's actually improved in Starcraft 2 compared with the original? Except graphics and marketing.

Although this is somewhat subjective, i will make a list of things that i like more on SC2 than on SC1. (Warning, spoilers might be found here!):

atmosphere - i feel a lot more drawn into the game, all those references to the original story are also very cool + the large amount of ingame cutscenes and even mini-movies are awesome (the one with Zeratul and Kerrigan is my fav so far)
micromanagement of units - far better. Great management of groups, no limit of units per group (as far as i know). The ability to schedule commands is the best improvement in the game regarding control of units. A base can be build in a few clicks and you can focus on the fight nearby without interrupting to get back to the base and build the next building.
upgrades - clever design of upgrades, often you have to choose which one you want because you cant have all. In most games, when you are rich you simply buy everything and thats it. Not possible here (talking about the campaign).
better feel of the races - even more different units and structures than in SC1
saving - automatic saves at key points of the mission are handy. Not sure if this was in SC1 too but i think it was not.
single player - playing against a computer is now much better customized so choosing easy gives you really and easy game. This was not the case in SC1 where even the easiest oponent was tough for newbies. This was sometimes really frustrating.

I think thats it, there are more things which i think are better, but they are really too subjective.

I feel that people have mentioned some of the differences, but from a Starcraft 1 perspective, a large number of things have been overlooked and I've tried to chronicle them here.

Worker rallies - one of the first things you will notice is the ability to rally workers to Minerals/Gas and they will immediately begin work (thus reducing your micromanagement)
Worker Idle - Idle workers can be selected with the F1 Key. This will select only 1 unit. To select them all you must hold Control and press F1.
Worker Splitting - Workers will automatically split off to unused mineral patches when sent to the same mineral patch. This reduces the need to do this process manually.
AI Pathing - Melee units will now automatically surround enemies and structures increasing their effectiveness
AI Targeting - Units will naturally target enemy units they are more effective against
Damage Benefits - Armor Types from Warcraft 3 are included as Light or Armored (additionally, no type and biological are effective types) many units do increased damage to one type or another.
Increased Damage - Every unit that is a holder from Starcraft 1 does more damage (with the exception of the Zergling who will do more damage with the Improved Pathing).
High Ground - In SC1 high ground provided a chance of damage immunity per shot for ranged weapons. In SC2 this is gone, however units from low ground cannot shoot high ground units without vision (this was not true in SC1)
Smartcasting - As mentioned before the closest unit will cast a spell to the target. If that unit is already acting on such an order, the next closest unit is used and so on. This also means that the closest unit in a group tends to be drained of energy fastest.
Reduced Overkill - This is the biggest boost to units like Siege Tanks. Previously if one Zergling ran at 10 tanks, all 10 would fire on it (despite a single one being sufficient). In Starcraft 2, these units will coordinate and only 1 will fire.
Centered Splash Damage - With the exception of the Thor (who fires projectiles) all splash units center their splash radius on the center of their target. This is in contrast to splash originating from the closest point as in SC1.
Macro Mechanic - As detailed here every Race in SC2 has a unique Macro Mechanic to increase both unit production and economy.
Creep - Where as Creep functioned as a building platform in SC1, in SC2 it also increases Zerg unit move speed
Burrow - Ultralisks can now burrow, as well as the new Roaches and Infestors. Roaches and be upgraded to move while burrowed and Infestors start with this ability. This effectively gives Zerg invisible units
Overlords are not detectors - They must now be upgraded to Overseers (50 Minerals, 100 Gas) to work as detectors
When your main worker-producing structure dies you are revealed after a short time
Doodad blockers - There are doodads with HP that function as obstacles on many maps.
Xel'Naga Towers - There are neutral towers which are controlled by having a unit close to them (workers in these positions are not considered idle) giving sight to the surrounding area (range 11).
Customizable Decals - This is an Aesthetic feature.
Maps Start Explored - While fog of war is still present, the minimap will show the features of a map as the start of combat. In SC1 the map was initially blacked out. The SC2 Campaign still has maps blacked out.
LoS Blockers - Certain doodads act as line of sight blockers though do not hinder the passage of units.
Camera Mapping - The F4 through F9 keys can be used to map certain camera positions in the same manner as control groups.
Control Group Size Increased - Control Groups can now consist of 255 units, though only 200 will be displayed. These will be shown in numbered subgroups of 24 units each. The control group will also show the number of units in it on the HUD.
Multiple Building Selection - Multiple buildings can now be selected in the same manner as units. They can be queued at the same time and rallied at the same time. They may even be placed in control groups with other buildings or even units.
Waypoint - A similar waypoint system to Warcraft 3 exists where multiple actions can be set up. This includes things like abilities, and can even be used on rally points.